A note left behind by Israel Regardie on the construction of the Vault walls offers an interesting approach to the toilsome task of mixing the appropriate colours for the numerous squares:
"The most practical plan is to have the walls of the Vault coloured as in the last diagram.
Then the way to differentiate between the various planetary sides of the Vault is simply to cover any particular side with a large sheet of strong cellophane or other plastic. So, for the Venus side, the wall could be covered with a Green sheet of plastic. For the Jupiter side with a blue sheet of plastic. For the Mars side with red. For the Saturn side with very dark blue or indigo. For the Solar side, use yellow or gold plastic. For Mercury a yellow orange plastic. And for Luna, a lavender or light violet plastic."
Putting aside the fact that the suggested colours here are wrong (the Jupiter wall should be violet, Sun should be orange, Mercury yellow, and Moon blue), this idea is a novel one, though not exactly one I find very appealing.
It "simplifies the task," as Regardie puts it, and cuts out a large volume of work required, since painting a Vault is an immense undertaking. The biggest issue with a Vault is not the construction of the room itself, but the mixing of the base colours of the squares with the base colours of the walls, resulting in a uniquely tinted assortment.
This is largely a trial or error affair, as the instructions left behind do not specify exact ratios of paint required, and this has led to existing Vaults looking sometimes very different from each other when it comes to colour tones. There is nothing wrong with this, as undoubtedly the original Vaults looked very different to each other, as each artists' work takes on, as it were, a hint of his or her character, or, indeed, the character of the Order or Temple in question.
The issue for me with Regardie's suggestion is that plastic as a material tends to take the magic out of a lot of things. We do not have to go over the top with our tools or talismans (the original members were quite fond of cardboard and coloured paper, for example), but plastic just does not cut it for me. It always feels like such a lifeless material and usually heavily reminds us of our present-day, factory-driven world, which does not exactly help with getting into the magical mindset.
I am also not particularly fond of the idea of having each wall's tint made purely from an overlaying material. I much prefer the idea that the colours are mixed to begin with to give that final hue, that you are not just looking at the same scene from a tinted window.
I suppose I am a little "old-fashioned" on this, but hand constructing and painting the Vault seems to me to be the best way to go, despite the huge investment of time and energy required. Since this is something for an entire Temple, or perhaps an Order as a whole, there will surely be at least one person involved who has enough artistic ability to do a good job with a paintbrush, and the work can be divided up to build a Vault more quickly.
That said, there are, of course, other options. Brodie-Innes suggested coloured papers. Printing is a growing trend in occult circles, which affords a potentially more accurate colour and a fancier print form of the symbols. Then there is Regardie's suggestion, and undoubtedly there are more ideas floating around for how best to undertake this task.